1. Field of the Invention
The present invention relates to a device for holographic storage.
2. Description of Related Art
As one of products of the modern storage technology, an optical disc has superiority in enormous storage capacity, convenience to store, long lifetime of data keeping, low cost and robust from stored data damage, and a user can easily read the data stored in an optical disc by using an optical disc drive. In general speaking, a laser beam is focused on an optical disc by an objective lens to read and record the data stored in an optical disc, and the storage capacity of an optical disc depends on the area thereof. With the same disc area, to increase the storage capacity thereof, it needs to increase the recording density of the optical disc, which however may encounter a diffraction limit as a bottle neck for a development in increasing the storage capacity thereof.
Accordingly, to increase the data-recording density of an optical disc, a holography technique is provided, which adopts a 3-D multi-recording operation and features in a mass storage. A general holographic storage medium mainly makes of a photosensitive material of photopolymer or photorefractive crystal for recording an interference pattern generated by a signal beam and a reference beam.
During reading a holographic storage medium, a reading beam which has the same condition as the reference beam is employed to re-establish the information recorded on the holographic storage medium. It needs to be noted however, during recording, a photosensitive material of photopolymer usually get a variation of volume in addition to the expected variation of optical behavior. Therefore, if the information re-establishment is based on the condition of the reference beam during recording without taking into account that the volume of the photosensitive material gets changed during recording, the light intensity representing the re-established information is reduced accompanying an increased noise, which leads to an increased probability of errors during reading the information. Besides, a holographic storage medium recorded under different conditions has different volume shrinkage factors, which lowers the interchangeability thereof between different reading devices and further causes an inconvenience for use.